1. Field of the Invention
The present invention relates to a system for controlling both an engine enabled to control output characteristics and temporary torque-down by changing an air/fuel ratio or the like and an automatic transmission connected to the engine.
2. Description of the Prior Art
In recent years, improvements in mileage of vehicles have been required from the points of preservation of the environment and problems in energy. For these improvements, there has been developed a lean combustion engine capable of running at a higher air/fuel ratio or an engine having its exhaust gases partially returned to the intake passage.
The lean combustion engine is constructed, as described in Magazine "Automobile Technology" Vol. 38, No. 9, such that an improvement in mileage under a light load and a high output under a heavy load can be made compatible by keeping the air/fuel ratio at a high level in a state having a predetermined throttle opening or less (i.e., under a light load). In case of running the lean combustion engine at a high air/fuel ratio, the combustion is stabilized by establishing intense swirls in the cylinder. In order to improve the combustion efficiency, moreover, a swirl control valve is provided, or the intake port is modified to have a special shape.
FIG. 17 is a diagram plotting the relations between the output torque and the throttle opening of an engine which executes the lean combustion in accordance with the throttle opening. Thick, solid curves appearing in FIG. 17 indicate the practical engine torques. Incidentally, curve 1 in FIG. 17 plots a torque characteristic at the time of a lean combustion, in which the air/fuel ratio is about "21". In this state, the a swirl control valve is closed. On the other hand, curve 2 or 3 plots a torque characteristic at the time of a rather lean combustion, in which the air/fuel ratio is about "17" or "16". Moreover, curve 4 plots a torque characteristic at the time of a stoichiometric combustion, in which the air/fuel ratio is at "14.5". In addition, curve 5 plots a torque characteristic at the time of an power combustion, in which the air/fuel ratio is at about "12.5".
In the aforementioned lean combustion engine, as shown in FIG. 17, for a throttle opening T.sub.A lower than a value T.sub.A1 of FIG. 17, the air/fuel ratio is set to about "21" to effect the lean combustion. On the other hand, in the state having the throttle opening T.sub.A between the values T.sub.A1 and T.sub.A2 of FIG. 17 (i.e., T.sub.A1 .ltoreq.T.sub.A &lt;T.sub.A2), the air/fuel ratio is gradually changed to about "17 to 16" to increase the engine torque continuously. When the throttle opening T.sub.A exceeds the value T.sub.A2, the air/fuel ratio is decreased to a power value simultaneously as the aforementioned swirl control valve is opened, to retain the torque in the large throttle opening range. In the combustion state having the swirl control valve opened, however, the engine torque discontinuously changes, as indicated between T.sub.1 and T.sub.2 in FIG. 17, with the change in the air/fuel ratio.
In the lean combustion engine described above, the output torque discontinuously changes with the change in the air/fuel ratio. This situation is also experienced by the engine of the type in which the exhaust gases are partially recirculated to the intake passage. In the automatic transmission connected to the engine of this kind, the torque to be applied to a frictional engagement unit seriously fluctuates before and after the output torque of the engine discontinuously changes. Thus, the durability of the frictional engagement unit is maintained, and the shift shocks are reduced by performing controls to change the servo oil pressure of the frictional engagement unit highly or to reduce the output torque of the engine at a shifting time.
The setting of a high air/fuel ratio and the much recirculation of engine exhaust gases will lead to a drop in the output torque of the engine. With a large throttle opening, however, a high torque is demanded, and for this demand, it is necessary to decrease the air/fuel ratio and to interrupt the much recirculation of exhaust gases. These controls are executed not only in case a high torque is demanded but also in case an engine water temperature is low. Specifically, the combustion in the cylinder in case of a low engine water temperature is not so smooth as that in case of a high water temperature. Thus, the air/fuel ratio is decreased to facilitate the combustion even with a small throttle opening. As a result, the engine output characteristics in case of a small throttle opening are at least two kinds: those in the state of a high engine water temperature and those in the state of a low water temperature. In the prior art, however, the shifting oil pressure of the automatic transmission at a shifting time is uniquely set according to the throttle opening. In case the engine output characteristics change into a plurality of kinds, as described above, any of them is mismatched by the shifting oil pressure of the automatic transmission. As a result, the automatic transmission may fail to have its shifting characteristics optimized.
On the other hand, let the case be considered, in which the aforementioned engine having a changing air/fuel ratio, e.g., the lean combustion engine is to be controlled. If the air/fuel ratio is changed from the stoichiometric to lean ranges as the engine water temperature rises, the engine has its output characteristics accordingly changed. If a shift occurs simultaneously with the change in the output characteristics, it obliges a control to drop the engine torque. On the other hand, if the air/fuel ratio is changed from the lean to stoichiometric ranges in case the throttle opening abruptly increases while the vehicle is running in a lean combustion state, a control is executed to drop the engine torque for a shift in accordance with the change in the throttle opening. In an EGR engine and an automatic transmission connected to this engine, on the other hand, the control of dropping the engine torque is also executed as in the lean combustion engine, if a shift occurs when the recirculation rate of exhaust gases is to be changed. Thus, the change in the engine output characteristics, the shift in the automatic transmission, and the drop of the engine torque accompanying the shift have to be premised to occur simultaneously. It is, however, difficult to time those three controls properly due to unavoidable delays in responses in the individual controls or other causes. As a result, the shift shocks may become serious, or the frictional engagement unit may excessively slip to have its durability deteriorated.